1. Field of the Invention
The present invention relates to the oxidative conversion of methane to higher hydrocarbons. In particular, the invention relates to a process whereby a mixed feed comprised of methane and molecular oxygen is contacted with a contact solid containing a reducible metal oxide at reacted conditions effective to form higher hydrocarbons and water wherein the reducible metal oxide is periodically contacted with molecular oxygen in the absence of methane at conditions effective to substantially fully oxidize the reducible metal oxide. By this process, the reducible metal oxide is maintained in a more active state during methane conversion.
2. Description of the Prior Art
This invention relates to the conversion of methane to higher hydrocarbons. A particular application of this invention is a method for converting natural gas to more readily transportable material.
Methane can be converted to higher hydrocarbons by reaction at conditions of elevated temperature--e.g., a temperature selected within the range from about 500.degree. C. to about 1000.degree. C. For example, methane can be contacted with an oxidative synthesizing agent containing a reducible metal oxide in the absence of gaseous oxidant at such elevated temperatures in order to produce higher hydrocarbons in a cyclic "redox" mode, the metal oxide being periodically reoxidized as with molecular oxygen. Reducible oxides of several metals have been identified which are capable of converting methane to higher hydrocarbons. In particular, oxides of manganese, tin, indium, germanium, lead antimony, bismuth, praseodymium, terbium, cerium, iron and ruthernium are most useful. See commonly-assigned U.S. Pat. Nos. 4,443,644 (Sb); 4,443,649 (Mn); 4,444,984 (Sn); 4,445,648 (In); 4,443,645 (Ge); 4,443,674 (Pb); 4,443,646 (Bi); 4,499,323 (Pr); 4,499,324 (Ce); and 4,593,139 (Ru). See also commonly-assigned U.S. Pat. No. 4,721,828.
Commonly-assigned U.S. Pat. No. 4,554,395 discloses and claims a process which comprises contacting methane with an oxidative synthesizing agent under elevated pressure (2-100 atmospheres) to produce greater amounts of C.sub.2 + hydrocarbon products.
Commonly-assigned U.S. Pat. No. 4,560,821 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with particles comprising an oxidative synthesizing agent which particles recirculate between two physically separate zones--a methane contact zone and an oxygen contact zone.
U.S. Pat. No. 4,499,322 discloses and claims a process for the conversion of methane to higher hydrocarbon and comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkali metal and/or compounds thereof.
U.S. Pat. No. 4,495,374 discloses and claims a process for the conversion of methane to higher hydrocarbons which comprises contacting methane with an oxidative synthesizing agent containing a promoting amount of alkaline earth metal and/or compounds thereof.
Hinsen and Baerns report studies of a continuous "cofeed" mode for the oxidative coupling of methane wherein regeneration air is cofed with methane feed. Hinsen, W. and Baerns, M., "Oxidative Koppling von Methan zu C.sub.2 --Kohlenwasserstoffen in Gegenwart unterschiedlicher Katalsatoren", Chemiker-Zeitung, Vol. 107, No. 718, pp. 223-226 (1983). Using a catalyst based on lead oxide and gamma-alumina in a fixed bed reactor operated at 1 atmosphere total pressure and 600-750 degrees C., they report results of approximately 53% selectivity to ethane and ethylene at 8% methane conversion for a feed consisting of about 50% methane, 25% air and 25% nitrogen. Other metal oxides studied by Hinsen and Baerns included oxides of Bi, Sb, Sn and Mn.
U.S. Pat. No. 4,523,049 discloses and claims a process for converting methane to higher hydrocarbons in a cofeed mode which comprises contacting methane and an oxygen-containing gas with a solid comprising a reducible metal oxide and an alkali/alkaline earth metal promoter.
U.S. Pat. No. 4,523,050 discloses and claims a process for converting methane to higher hydrocarbons in a cofeed mode which comprises contacting methane and an oxygen-containing gas with a manganese silicate.
Commonly-assigned copending U.S. Pat. No. 4,788,372 discloses and claims a method for converting methane to higher hydrocarbons wherein methane and added water are contacted in the substantial absence of added gaseous oxidant with a solid comprising at least one reducible metal oxide.
Commonly-assigned U.S. Pat. No. 4,801,762 discloses and claims a method for converting methane to higher hydrocarbons wherein methane and a gaseous oxidant together with added water are contacted with a non-acidic solid and/or a reducible metal oxide.
Other relevant patents include U.S. Pat. Nos. 4,777,312, 4,795,849 and the like.
The reaction products of the foregoing processes are hydrocarbons, carbon oxides, coke and water.
The methane conversion reaction can be carried out by contacting methane with the reducible metal oxide in the fluidized bed reaction systems as well as in fixed bed systems.